Nutritional value and bioaccumulation of heavy metals in nine commercial fish species from Dachen Fishing Ground, East China Sea

The study evaluated the nutritional quality in muscle tissues of nine commercially important marine fish species. And the concentrations of trace metals (i.e. As, Hg, Cu, Pb, Cr, Cd and Zn) in the muscles (edible part) and tissues (gill and liver) of fishes caught from Dachen fishing ground, the coast of Zhejiang Province, East China Sea, were determined, and the values of target hazard quotient (THQ) and the carcinogenic risk (TR) were calculated for assessing human health risk. Significant differences(P < 0.05) were observed in the proximate chemical composition of fish muscles in these species. The muscle protein content of fish species ranged from 12.36 to 23.41%. The muscle lipid content of fishes ranged from 0.48 to 2.54%. The accumulation capacity of heavy metals (except Cr) in livers and gills was higher than that in muscles. In addition, the accumulation ability of most fishes is related to the water layer they live, the fishes living in the demersal layer showed more accumulation of heavy metals than the middle-upper layer(except Cu). Estimated daily intake (EDI), target hazard quotient (THQ), hazard index (HI) and the carcinogenic risk (TR) assessed for potential human health risk implications suggest that the values were within the acceptable threshold for human. However, the carcinogenic risk(TR) of As and Cr was close to the critical limit (10–4). Therefore, in order to ensure the health and safety of human consumption, the continuous monitoring of heavy metals in Dachen fishing ground area is suggested.


Material and methods
Study areas and sample collection. This study was carried out in the sea area of Dachen Islands (28° 28′ 12.00″ N-28° 22′ 12.00″ N, 121° 48′ 00.00″ E-121° 60′ 00.00″ E) (Fig. 1). The selection of fish species in this paper mainly considered economy and catch. And they represent different water layers. Samples of 9 different fish species were caught by bottom trawling in November, 2019 (Fig. 2). Also they are carnivorous and have the potential to bioconcentrate contaminants, which would normally be present in the water or inside sediments. After the fishing ban, the biomass and biodiversity of the fishes are high in the sea area of Dachen Islands. For each species, six samples were collected for this study. The total length and weight for each individual was measured and recorded in Table 1. Sufficient amount of muscle, liver, gills from the fish of the same species were removed from each organism and dissected on-site by clean stainless steel knife. It should be mentioned here that only edible part (fish flesh and skin) were chosen as muscle tissue. Each collected tissue sample was preserved in clean plastic bags and frozen immediately until it was transported to the laboratory. The tissues samples of each individual fish were then air-dried to remove the extra water, and subsequently were dried at − 80 °C for 24 h using a vacuum freeze-drying instrument (Christ Alpha, Germany), rations of moisture were also accurately calculated by comparing the weight before and after drying. Then ground to powder, stored at -20 °C for succeeding uses.
Seawater samples for heavy metal determination were collected in acid washed polyethylene bottles. The bottles were rinsed thoroughly with deionised water after being washed in dilute nitric acid (HNO 3 ). In the field the bottles were rinsed several times with the seawater and 1 L of water sample was then collected at about 50 cm below the water surface. And seawater samples were filtered through Whatman No. 45 filter paper. The seawater samples were acidified with concentrated nitric acid for preservation. The acid pretreatment ensured that heavy metals did not get absorbed to the surface of the container during transportation and storage.
A total of 10 surface sediment samples were collected at sampling sites distributed throughout the study area ( Fig. 1). A stainless-steel grab sampler was used for collection, while a plastic shovel was used to excavate the sediments from the middle part of the sampler. Samples were kept in pre-cleaned polyethylene bags and frozen until lab analysis.
Sample analysis. The proximate composition of fish muscles (protein and lipids) was determined according to the standard methods of AOAC 16 . The metal extraction procedure is referred to a previous study 17 and we have made corresponding improvements based on this research. Both fish and sediment samples were weighed in duplicate. For fish and sediment samples, about 0.5 g of each powdered sample was digested in microwave digester (MDS-6G, China) using the ratio of 2:1 (8 mL + 4 mL) of concentrated nitric acid (GR) and hydrogen peroxide (AR). After digestion process, one of them does not removed acid to determine arsenic and mercury. And the other transferred the sample to a 30 mL polytetrafluoroethylene beakers and heated it on hot plate at 170 °C until about 1 mL of solvent remains to determine other heavy metals. Then samples were diluted with  www.nature.com/scientificreports/ deionized water in 25 mL polypropylene tubes. The concentration levels of Cu, Zn, Pb, Cd and Cr in the digested sample solutions were determined by using inductively coupled plasma optical emission spectrometer (ICP-OES, Optima8000, USA), and the Hg and As contents were determined by using atomic fluorescence spectrometry (AFS-3100, China). Certified Reference Material (Standard oyster Tissue 1566a) obtained from the U. S. Department of Commerce was also analyzed routinely every 10 samples and all runs were analyzed in triplicated as quality control. In addition, reagent blanks were analyzed to provide a baseline correction for the results of the samples. The mean recoveries of metals were between 91. 8  Statistical analyses. Data were given as mean ± SD for each of the measured variables. All statistical analyzes were performed using SPSS version 21.0 version. The one-sample Kolmogorov-Smirnov (K-S) test was used to assess the data normality. All the concentration values for seven heavy metals in the tissues of fish species were normally distributed at the 95% confidence level. Therefore, a one-way ANOVA test was used to assess significant differences tissues. Turkey's Post hoc test was employed to test the significance of difference between single species in different organ.
Health risk assessment. Several methods have been proposed for estimating the potential risks to human health of heavy metals in fishes [19][20][21] . These methods were described in detail below. Mean concentrations of heavy metals for each species were used in all calculations.  www.nature.com/scientificreports/ Estimated daily intake (EDI). Health risk was estimated considering the average concentrations of all fish muscles and daily heavy metal intake (EDI). The specific formula is as follows 9 : where C is the concentration of heavy metals in the selected fish tissues (mg/kg, ww); FIR is the food ingestion rate, which is 31 g/person/day 22 ; ED is the exposure duration 70 years 23 ; EF is the frequency of exposure 365 days/ year 23 ; BW is the average body weight 55.9 kg 24 ; AT is the average exposure time 25,500 days 9 .
Non-carcinogenic health risk. The target hazard quotient (THQ) and hazard index (HI) are a method proposed by the U. S. Environmental Protection Agency (USEPA) for assessing the risk of heavy metals caused by food intake by the human body 23 . The value of ratio < 1 implies a non-significant risk effects. The specific formula is as follows 23 : RFDs of the different heavy metals for example As, Hg, Cd, Cu, Zn, Pb, and Cr are 0.0015, 0.00016, 0.001, 0.04, 0.3, 0.004 and 1.5, repectively 23 . Other factors have been mentioned as above.
The carcinogenic risk (TR). As well as non-carcinogenic risks, there are also carcinogenic risks in human health risk assessment 17 . All trace metals do not have carcinogenic effects. However, As, Pb, Cd and Cr among the studied heavy metals are considered as carcinogens. For carcinogens, the individual risk assessment increases the probability of developing cancer due to exposure to potential carcinogens 22,25 . The acceptable risk levels of TR 5,9 for carcinogens ranged from 10 -6 to 10 -4 . The model formula is as follows 26 : where the oral carcinogenic slope factor (CSFo) was obtained from the database of the U. S. Environmental Protection Agency 26 . Available CSFo values (mg/kg/day) are: As (1.5), Pb (0.0085), Cd (6.3) and Cr (0.5) 26 . Other factors have been mentioned as above. Assume that 10% of the total As can be assessed as inorganic state 5,27,28 in this study.
Ethical approval. Fish used for this study were fresh but lifeless, however all procedures used conform to standard scientific research guidelines. All methods are reported in accordance with ARRIVE guidelines. All experimental protocols were approved by the Shanghai Ocean University. The study was approved by the ethic committee of the Shanghai Ocean University.

Results
Chemical composition of fish muscles. The chemical composition of fish muscles is shown in Table 2.
The muscle protein content in Larimichthys crocea (23.41%) was significantly higher than that in the other species, followed by Muraenesox cinereus (21.28%) whereas the muscle protein content in Ctenotrypauchen chinensis (12.36%) was the lowest. The muscle lipid content of Sebastiscus marmoratus had significantly higher lipid content (2.54%), while Ctenotrypauchen chinensis had significantly lower lipid content (0.48%). And compared with other fishing grounds, the muscle protein content of fishes basically was the same ( Table 2). The muscle lipid content of fishes ranged from 0.48 to 2.54% and lower than the reported levels in other fishing Ground ( Table 2).

Heavy metals in fish samples. The content of heavy metals in fish muscles.
In the present study, the concentrations of seven heavy metals (As, Hg, Cd, Cu, Zn, Pb, and Cr) in the muscles of nine fish species were shown in Table 3. Metal concentrations were reported using wet-weight. The concentration of all the examine heavy metals in fish muscles remained well below the acceptable limits for human consumption established by FAO. The order of mean concentrations of heavy metals from high to low was shown as follows: Zn (16.910 mg/ kg) > Cu (2.810 mg/kg) > As (0.301 mg/kg) > Pb (0.264 mg/kg) > Cr (0.074 mg/kg) > Cd (0.067 mg/kg) > Hg (0.044 mg/kg). It was found that the accumulating capacity of most fishes was linked to the water layer in which they lived (Fig. 3). The results obtained from this study were analyzed using the analysis of variance (ANOVA), and the differences between essential (Cu and Zn) and non essential metals (As, Hg, Cd, Pb and Cr) concentration levels were considered significant at 95% confidence interval (P < 0.05). The amount of heavy metals in the demersal fish was greater than the middle-upper fish (except Cu).

Distribution of heavy metals in different tissues of fish.
The concentrations of seven heavy metals in various tissues of nine marine fish species were shown in Fig. 4. In this study, the accumulation of heavy metals in most fish was higher in liver or gills than in muscle. The content of Zn in the liver of Muraenesox cinereus was the highest (77.410 mg/kg). And the content of Zn in muscle of Muraenesox cinereus was also relatively high. Moreover, the www.nature.com/scientificreports/ www.nature.com/scientificreports/ content of Hg in the liver of Johnius belengeri was the lowest (0.006 mg/kg). The accumulation ability of heavy metals varies among different tissues in the same fish. Our data indicated that the mean concentrations of heavy metals in muscle, liver and gills differed significantly (P < 0.05) in most fish species. The heavy metal Cu content in the liver and gill of Muraenesox cinereus exceeded that of the muscle. The accumulation ability of Cu in other fish was similar to Muraenesox cinereus. However, based on our results, it was found that the ability to accumulate Cr was different from that of Cu. Other heavy metals have the same accumulation capacity as Cu.

Health risk assessment. Estimated daily intake (EDI). The EDI values of seven heavy metals calculated
for nine fish species were given in Table 4. The EDI value of Zn in all fish species were found higher, while those of Hg in most fish species were found lower. The EDI value of seven heavy metals calculated for each fish species was very lower than the tolerable daily intake (TDI) value, which suggested that the daily intake of seven heavy metals via the ingestion of nine fish species in this study would pose no detrimental health risk to humans.
Non-carcinogenic health risk. The estimated target hazard quotient (THQ) for individual heavy metal from the consumption of various fish species was shown in Table 5. The THQ results for seven heavy metals in nine marine fishes revealed that THQ values for all metal in nine fishes were below one, and the THQ values of different heavy metal of the same species were quite different. And, hazard index (HI) values of combined heavy metals calculated for each fish species were below one. The THQ and HI results suggested that non-carcinogenic health risk from the intake of individual or combined heavy metals in the fish species were not expected for consumers.
The carcinogenic risk (TR). Due to the high toxicity of As, Pb, Cd and Cr the target cancer risk (TR) for these four heavy metal elements was estimated and the results were shown in Table 6. The measured TR values of As, Pb and Cd were ranged from (2.33-8.15) × 10 -5 , (0.01-0.52) × 10 -5 , (4.68-4.75) × 10 -5 and (5.96-7.85) × 10 -5 , respectively. The results showed calculated TR values were within the risk as acceptable range is 10 -6 to 10 -4 . From the point of As, Pb, Cd and Cr TR values, the fish will not cause carcinogenic effects on humans through food consumption. Among them, two fishes (Sebastiscus marmoratus and Johnius belengeri) were higher TR values of Pb than other fishes, which should be given a great concern.

Discussion
In this study, the amount of muscle protein in fish species ranged from 12.36 to 23.41%. The study reported that the higher protein content observed in the fish was due to its feeding habits 43 . The protein contents of varies fish were found to be possibly due to their food availability 44 . And no correlation was observed between protein contents and heavy metals in fish muscle (using Pearson correlation test, P < 0.05). Studies 45 also had shown that there was no correlation between heavy metals and protein content in muscle tissue. This is consistent with our study. And the muscle lipid content of fish ranged from 0.48 to 2.54%, where it varied significantly. Younis et al. 46 stated lipid content is affected by feeding habits and the territorial food. Nath et al. 47 reported that lipid content is influenced by the life cycle and environment. In the present study, no correlation was observed between lipid contents and heavy metals in fish muscle (P < 0.05). The differences in the chemical of marine fishes in this study may be due to difference in species, feeding habits, and this agrees with the study 46,48 . This study found that the content of essential micronutrients (Zn, Cu) is higher than that of non-essential micronutrients (Pb, Cd, As, Hg, Cr). It might because the automatic adsorption of Zn and Cu is processed by organisms, which causes the content of Zn and Cu in fish muscles is greater than that of Pb, Cd, As, Hg and Cr. Zn is a component of a variety of enzymes in the body, and Cu is a component of various oxides in the body 49 .
The amount of heavy metals in the demersal fish was greater than the middle-upper fish (except Cu). This was consistent with the research by Sun et al. 50 . Demersal fish were easily exposed to heavy metals from sediments,   www.nature.com/scientificreports/ which were seen as the main source of heavy metals in marine fish 51,52 . The content of Hg in fish muscle living in the demersal was significantly higher than that of the middle-upper layer. UNEP 53 research showed that marine fish mercury is methyl mercury, mainly from the marine environment and the food chain transmission. Methyl mercury is primarily caused by the biological and abiotic methylation process of inorganic mercury 54 . Especially, the process of mercury bioremediation on the surface sediment might resulted in the bottom seawater with higher levels of methyl mercury. The corresponding mercury content of demersal fish was greater than that of middle-upper fish. The mercury content of heavy metals in water and sediment was simultaneously detected. The results suggested the mercury content in the sediment was higher than that in the seawater. However, Cu in demersal fish was lower than that in the middle-upper. The research by Yang et al. 55 had also similar founding, the bioavailability of heavy metals in fish with varied habitats being different. The accumulation ability of heavy metals in different tissues of the same fish was different. Our data indicated that the mean concentrations of heavy metals in muscle, liver and gills differed significantly (P < 0.05) in most  Table 5. The target quotients(THQ) and hazard index(HI) established for the consumption of fish from Dachen fishing ground, East China Sea(ww).  www.nature.com/scientificreports/ fish species. Different fish have different ability to accumulate heavy metals. This may be linked to the lifespan of fish in the water body and the difference in physiological metabolism 56 . This was in line with the research by Monroy et al. 57 on the concentration and distribution of heavy metals in fish. It indicated that the liver and gills were the main organs for heavy metal accumulation in fish. The unique structure of the gills is conducive to the penetration of ions in the water, making the gills the central part of fish to directly absorb heavy metals from the water environment 58,59 . The enrichment of heavy metals in the liver was mainly associated with the induction and bonding of metallothionein. The liver is a tissue that continually accumulates, biotransforms, and detoxifies. Therefore, measuring the content of heavy metals in the liver was beneficial to assess the level of heavy metal pollutants in the environment 60,61 . This demonstrated that the primary means of heavy metals entering fish include food intake, branchial membrane adsorption, liver digestion and absorption, and accumulation. However, based on our results, it was found that the ability to accumulate Cr was different from that of Cu. Generally, the content of Cr in muscle was higher than that of the gills and the liver. Since the low absorption rate and a relatively high excretion rate of Cr, the retention rate of Cr in fish is low 62 , resulting the reduction of Cr to some extent in the liver and gills. Because the content of heavy metal As in marine organisms is generally relatively high, in marine organisms, arsenic occurs mainly in the fasts of organic compounds, as arsenobetaine 27 . Its toxicity is expected to be very low, and it can be quickly eliminated from the body after swallowing. The toxic inorganic arsenic generally only accounts for 0.1-10% of the total arsenic content 27 . Assume that the maximum rate of 10% was used to estimate the level of inorganic arsenic content in fish in the Dachen Sea. In this case, the As content of all the samples in this study was comparatively low. The source of arsenic pollution may be domestic sewage and the use of arseniccontaining pesticides 63 . However, mean arsenic content (0.301 mg/kg) of this study differs from that of Peng et al. 14 (1.600 μg/g, total arsenic). Future studies should address the As forms accumulated in the edible muscle.

Species
Moreover, Pb is toxic to organisms, can disrupt normal metabolic activities, transmitted through the food chain, enriched and accumulated, and can be transformed into more toxic organic compounds under certain conditions 64 . Lee et al. 65 had found that long-term exposure to high levels of Pb can damage the brain, liver and kidneys and even reduce the function of the nervous system, eventually leading to death. This study found that the enrichment trend of Pb was similar to that of Cu, and its content in the gills and liver was higher than that muscle. The average content of Pb (0.264 mg/kg) in this study was consistent with that of Li 66 (0.128 mg/kg) but different from that of Peng et al. 14 (5.700 mg/kg). It may be caused by the development of coastal industries in Zhejiang province over the past few years. The concentration of Pb in the aquatic environment is higher, which exceeds the fish body's discharge capacity, resulting in a parallel accumulation of Pb in the fish body (gills, liver, muscle). Studies 63,67 have reported that the industrial wastewater and domestic sewage generated by the pharmaceutical, chemical and electronic electroplating industries in the Jiaojiang Estuary water were directly discharged into the water, resulting in high lead content. Dachen Island is located in the southeast of Jiaojiang River estuaries and is highly susceptible to lead pollution from the Jiaojiang River estuaries. Besides, due to the development and utilization of fishing and ship transportation in the Dachen sea area in recent years, the wastewater produced by the local maritime transport has also caused lead pollution to a certain extent.
The heavy metal pollution is one of important environmental factors that can considerably affect human health. The muscles of fish may enter into human metabolism through food consumption, leading to serious health risks 68 . Therefore, this study assessed the impact on human health by studying heavy metal pollution in fish in the Dachen Island region. Moreover, it is also conducive to the construction and management of marine ranches 10 .
The EDI reflects the daily exposure to the heavy metal, and is executed to avoid any harmful effect on human health 28 . The EDI values lower than TDI guidelines suggested a lower possible health risk of the heavy metals to the consumers. However, it would not be wise to take it as a permanent measurement to reach a final conclusion 28,69 .
The THQ and HI values below one. There was no potential non-carcinogenic effect for the consumers due to intake of the fish species. Studies carried out by several authors in similar conditions were in line with our results 7,9,28 . However, due to multiple simultaneous pollutants, human could dramatically suffer in the long run 70 . The THQ values of different heavy metal of the same species were quite different. It was found that the risk of Cr was relatively low. Many kinds of literature 71,72 have suggested that the potential risk of Cr was low, which was also proved by this study, and the potential health risk of Cr was the lowest, which may be ascribed to its higher RFD value.
The results showed calculated TR values were within the risk as acceptable range is 10 -6 to 10 -4 , and consumers were less prone to carcinogenic. In fact, 90% of the carcinogenic risk is observed in the As contaminated acquatic food items 28 . The inorganic form of As is more lethal than organic one 27,73,74 , and only 10% of the total As can be assessed as inorganic state 5,28 . In this study, although the TR value of As was within an acceptable range, regular monitoring of fish in this fishery was still essential.
In this study, we assumed that the intake of heavy metals was equivalent to the absorption of heavy metals, without considering the time and residual content of heavy metals in the human body. This hypothesis improves the calculation of carcinogen risk. Some heavy metals may be excreted. The heavy metals levels in blood, and urine are suggested to be measured in the future.

Conclusion
In this study, the results showed that all nine fish species were a good source of protein and lipids. The concentrations of seven heavy metals in various tissues of nine marine fish species were compared and found to be varied considerably among tissues and species. Maximum and minimum of heavy metals concentrations in the fish respectively were determined as Zn and Hg. The accumulation capacity of heavy metal (except Cr) in the liver www.nature.com/scientificreports/ and gills was higher than that of muscle. The cumulative capacity of heavy metals in fish muscle was linked to the water layer they live, that was, the demersal fish contained more heavy metal than the middle-upper ones (except Cu). From the perspective of human health, the EDI of each element was lower than the respective recommended tolerable daily intake. The THQ and HI values of fish indicated that fish in the Dachen fishing ground were safe to eat. From the point of As, Pb, Cd and Cr TR values, the fish may not cause carcinogenic effects on humans through food consumption. However, the carcinogenic risk (TR) of As and Cr was close to the critical limit (10 -4 ). Therefore, regular and long-term monitoring of the heavy metal content of fish in this fishery is recommended.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request. www.nature.com/scientificreports/